Field photo Set #3

The newest pair of field photos are quite local to me, as they are a slice of glaciomarine layer cake left over from the last waxing of the Cordilleran ice sheet, specifically the Wisconsonian. The following pair show exposures of a glaciomarine clay unit sandwiched in-between granite bodies and Holocene soil & sediment, found within North Vancouver's uplands. What you'll see are portions of the Capilano Sediments member of the Sumas Drift group formed in the Quaternary.

The first photo (at right) is a classic exposure within the Capilano Canyon park & suspension bridge (49° 21.470'N 123° 06.698'W). You'll notice the beige coloring of clay that is packed into mud; this unit is quite distinct from surrounding Holocene soil, the greenery, and the granodiorite bedrock. It has a very smooth consistency, and upon scrutiny has micro-level faults from minor stresses placed on it. The layer dips a good 10-12º to the SE, with a NE-SW strike.

The foliage around the site is quite dense, even in the winter. Biomass detritus litters the photo, which was taken in the late winter months. This Pleistocene marine clay was formed at the front of valley glacier lobes stemming from the Cordilleran ice sheet. The continental slope at the front was relatively high, so deep sea marine deposition was not dozens of kilometers from the glacial terminus, but rather a few kilometers. Post-Pleistocene isostatic rebound was quick in the region, and it elevated this layer to at least 50m asl, and in some areas up to a few hundred meters. Post Eocene tectonic uplift of the coast mountains caused the northern edge of these types of sedimentary units to be gently warped upwards to the north and draped over the lower slopes of the older rocks of the north shore mountains.

This exposure of the same member is found along the Lynn Peak trail (49° 21.735'N 123° 01.519'W), about a quarter of the way up, roughly 300m asl. This portion is mantled by Holocene soil, and surrounded by a lot of till and colluvium. This particular photo shows the most distinct boundary surface between the clay and surrounding members. It is likely a lens of material formed during episodic activity (see link on Surficial and Bedrock geology). The continuity of the glaciomarine member is hard to follow along the north shore, as it is buried under steepland sediments, landslide debris, and thick vegetation. So spotting a slice of the cake is considered good luck for intrepid geological explorers.

Additional Info:

• GSC - Geomap Vancouver
• James Roddick (Caltech) - Some Features of the Geology of the North Vancouver area
• GSC - Surficial and Bedrock Geology of the Fraser Lowlands and Coast Mountains near Howe Sound

To study or not to study: a Metamorphic Genesis or a Dynamic Creek?

Lately I've been thinking about what topic I should choose for doing my senior undergraduate major project. This project is essentially worth 3 credits, or one full course. The synopsis outlines it as almost a thesis but with a narrower scope of time, and only to be peer reviewed by my university professors. I'm planning on it for this semester next year, but want to utilize the coming summer months for field work.

The first idea that comes to mind is to measure/analyze something local. I don't want to synthesize and regurgitate some phenomenon that is a world away where I can't put my hands on it (I do that enough on this blog). After all, one of the big reasons I chose geology is because on occasion the laboratory is the great outdoors. The second thing that comes to mind is that I want to break new ground. Not by finding new analytical methods or a stunning new hypothesis (I won't have the time, funds, knowledge nor experience to do that for several years), but by covering something in greater detail than has been.

I might have access to basic hydrological equipment: current meter plus pH, conductivity, and DO probes. In that case I wish to study the fluvial geomorphology of a particular large creek that has not had a thoroughly holistic study. Plus my favorite geology book is Leopold's, and this would give me an excuse to read it again. Its geomorphology is of particular interest because the creek cut a channel down through glacial till and exposed a volcanic basalt layer underneath, thus a small portion of its mid-reaches has step-pool falls. More details found in the earthcache I created for it.

Cliff falls in Maple Ridge, BC. Kanaka creek pinches
here due to eroding down to a resistant lava flow

If access to equipment or most of the drainage basin is not possible, I am considering doing a petrological study of a pre-Jurassic [mostly] metamorphic group on the north shore. There is surprisingly little information about it, beyond a few journal papers making slight references to it when describing other groups/formations that interact with it. This would involve a thorough investigation of approximately 8 sites of varying size (see map below) where outcrops have been identified, plus I'd get to use snazzy terms like aureoles and metamorphic facies. All I'd need in that case is a rockhammer, a handlens, maybe a petrographic microscope.

Surficial geology map of North & West Vancouver
#1 is the Twin Islands Group I'm considering for study

I'll likely put this on the back burner until late spring, when I will have time to investigate the creek and gauge how I can approach it from various access points. In the meantime, professorial advice and anecdotes will likely sway me towards one or the other.


Terminology note: It took me a while to wrap my head around the phenomenon of roof pendants. Sometimes a blatantly obvious mechanism or concept eludes me, until a light bulb goes on. This has happened with things like pedimentation, cyclothems, Benioff zone.  My best attempt to explain to myself the idea of roof pendants - When a batholith forms below overlying strata, it penetrates upwards, and thus makes contact with those layers. The layers will likely undergo contact metamorphism, and metamorphose under high temperature & low-moderate pressure. Erosion of subsequent material leaves the META members as isolated crops dangling/resting above the batholith. Essentially roof pendants have a parent rock, and are akin to xenoliths on a large scale but did not get included into the igneous mass, simply resting above it instead.

Additional Info:

• Landslide inventory in a rugged forested watershed: a comparison between air-photo and field survey data
• A Geological Excursion Across the Canadian Cordillera Near 49° N 
• Geology Vancouver North, British Columbia
• Kanaka Creek earthcache